1. Field of the Invention
The present invention relates to a brake oil pressure control device of a vehicle, more particularly, it relates to a brake oil pressure control device which can carry out an antiskid control upon braking and a wheel traction control upon acceleration. The antiskid control prevents the wheels of a vehicle from locking when the brake system of the vehicle is operated, so that drivability of the vehicle is maintained, and the acceleration slip control prevents loss of traction (hereinafter referred to as "slip") by the drive wheels when the vehicle is suddenly accelerated from a stop position, so that the running stability and acceleration ability of the vehicle are improved.
2. Description of the Related Art
Conventional brake oil pressure control devices which can carry out both the above antiskid control and acceleration slip control are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 57-22948 and 58-16948.
In the control device disclosed in the former publication, a first control valve selectively connects wheel cylinders to a main pipe which communicates with a master cylinder, or to a return pipe. A second control valve selectively connects the return pipe to a discharge pipe communicated with a reservoir, or to an auxiliary pipe communicated with a pump. When an antiskid control is effected, the second control valve connects the return pipe to the reservoir, and the first control valve connects the wheel cylinders to the main line or to the return pipe, so that pressure in the wheel cylinders is controlled. When an acceleration slip control is effected, the first control valve connects the wheel cylinders to the return pipe, and the second control valve connects the return pipe to the auxiliary pipe or to the discharge pipe so that pressure in the wheel cylinders is controlled. Namely, the return pipe is used to return brake oil in the wheel cylinders to the reservoir during the antiskid control, and to supply pressurized brake oil from the pump to the wheel cylinders during the acceleration slip control. Therefore, the return pipe is subjected to a high pressure, and thus the construction thereof must be able to withstand such a high pressure. Further, the second control valve must be constructed in such a manner that a perfect seal is effected between ports of the valve and pipes.
In the control device disclosed in the latter publication, an acceleration slip control device includes rotation sensors for sensing wheel rotation, and control valves for controlling brake oil pressure in the drive wheels. This acceleration control device further is provided with an electric control circuit, so that the drive wheels are controlled according to signals from the rotation sensors. The electric control circuit is provided with a signal output means which outputs a signal representing a slip value and a signal proportional to a vehicle velocity. A comparator in the electric control circuit compares the slip value with a threshold value which corresponds to a maximum value of the amount of slip allowed at the drive wheels. If vehicle speed is higher than the threshold value, a signal is output to reduce the drive torque of the engine. In this conventional brake oil pressure control device, a pressure control device must be provided only for acceleration slip control, and therefore, not only is the construction of the brake oil pressure control device complicated but also the size and weight of the device are increased.